1. Field of the Invention
The invention relates generally to the detection of pin layer reversal (PLR) in a disk drive using a magneto-resistive head and also to signal degradation, for example, damage to the disk drive, and/or domain wall movement of information recorded in the servo portion on the disk.
2. Background Information
A magneto-resistive head used in a disk drive includes a pinned layer of magnetic material which ideally has a fixed direction. As the magnetic domains on the disk pass under the head, the direction of the magnetization of each domain either adds to, or subtracts from, the magnetization of the pinned layer. The head senses the direction of each domain by means of a change in resistance of the head in response to the sum of the magnetization of the domain and that of the pinned layer. The direction of the pinned layer magnetization is set at the factory and the decoding of data on the disk drive is determined by that direction. That is, the direction is assumed in the circuitry that detects and decodes the data.
However, from time to time, the magnetization of the pinned layer may reverse, due to various causes. This results in a reversal of the perceived bits on the disk, i.e. zeros are interpreted as one's and one's interpreted as zero's. There is a need to detect pinned layer reversal. Also there is a need to detect noise, for example noise introduced by damage to the head or disk, or by domain wall movement that may occur in the soft under layer used in perpendicular recording systems.